1. Field
This invention relates generally to voltage detector circuits, and more specifically to a low power consumption, low voltage detector circuit.
2. Related Art
Electronic devices frequently employ a voltage detector to indicate if a particular voltage is outside a specified range. For example, a battery operated device often includes a voltage detector to provide a low voltage indication when the voltage supplied by a battery decreases below a predetermined level. The low voltage indication can notify a user of the device that the battery should be replaced or recharged. Usually, a reliable low-power, low voltage detector consumes hundreds of microamperes because it requires one or more voltage dividers and a bandgap reference voltage to operate properly.
There are current-based, low voltage detectors, but they require nonstandard CMOS devices such as bipolar transistors and non-volatile memory (NVM) cells.
Some known low voltage detectors use ladder resistors to generate voltage taps needed to detect a trip point. The ladder resistors are usually large so that known low voltage detectors consume low power. Large ladder resistors may cause such known low voltage detectors to occupy a disadvantageously large area.
With some known low voltage detectors, the trip-point depends on a threshold voltage Vth of a metal oxide semiconductor field effect transistor (MOSFET). The threshold voltage of a MOSFET depends upon process, and the threshold voltage changes greatly with temperature. When a low voltage detector uses a reference voltage that depends upon the threshold voltage, a part-to-part variation of the low voltage detector is disadvantageously great. Therefore, such a low voltage detector is either disadvantageously affected by temperature changes, or it requires additional circuitry that provides temperature compensation but which consumes additional power. Moreover, such a low voltage detector may need trimming to counterbalance the part-to-part variation.
Using the Advanced Compact Model (ACM) for a MOSFET, the inversion level of the MOSFET is determined by an inversion factor if, which is defined as if=I/Is, where I is a drain current of the MOSFET, and Is is a normalization current. The normalization current Is is equal to ISQS, where ISQ is a sheet specific current that is defined by certain process parameters and S is an aspect ratio of the MOSFET. The aspect ratio S of the MOSFET is a ratio of channel width W to channel length L. Furthermore,ISQ=nμC′ox(ΦT2/2)
where μ is a mobility of the carriers in the channel, n is a slope factor, C′ox is an oxide capacitance per unit area of the gate of the MOSFET, and ΦT is a thermal voltage.
The thermal voltage ΦT is a function of temperature and increases directly proportionately with increasing temperature. The thermal voltage is ΦT=kT/q, where T is the temperature measured in kelvins (abbreviated “K”, and sometimes informally referred to as “degrees Kelvin”), and q is the magnitude of the electrical charge of an electron (1.6022×10−19 coulombs). The Boltzmann's constant, k, can be expressed as 1.3807×10−23 joules per kelvin. The thermal voltage ΦT is approximately 25.85 millivolts at room temperature (approximately 300K). At room temperature, the thermal voltage ΦT changes at a rate of approximately 0.086 millivolts per degree Celsius.
In ACM, the relationship between current and voltage are given by:
                    V        P            -              V        S                    Φ      T        =                    1        +                  i          f                      -    2    +          ln      ⁡              (                                            1              +                              i                f                                              -          1                )            
and
      V    P    ≅                    V        GB            -              V                  T          ⁢                                          ⁢          0                      n  
where VP is a pinch-off voltage, VS is a voltage at a source terminal, VGB is a gate-to-bulk voltage, and VT0 is a zero bias threshold voltage.
Weak inversion, moderate inversion and strong inversion describe different operational modes of a MOSFET. Weak inversion occurs when a drain current of a MOSFET transistor is dominated by a diffusion current, moderate inversion is when the drain current has both diffusion current and drift current components, and strong inversion is when the drain current is dominated by the drift current. In a MOSFET, inversion occurs when a thinner channel is formed in the transistor in the substrate region under the gate. When there is no channel, the transistor is at cut-off. As a rule of thumb, a MOSFET that has an inversion factor of less than “1” is said to be in weak inversion; a MOSFET that has an inversion factor of about “1” to “100” is said to be in moderate inversion; and a MOSFET that has an inversion factor of greater than “100” is said to be in strong inversion.